The present invention relates to a method for manufacturing a channel polysilicon film of a thin film transistor (hereinafter referred to as "TFT") for a static random access memory (SRAM) in a highly integrated semiconductor device, and more particularly to a method for enlarging the grain size of a channel polysilicon film of a TFT.
In order to meet requirements of high-speed operation and low power dissipation accompanied with contriving high packing density in SRAM devices, research & development on PMOS TFT-type load cells has been rapidly carried out. This is because the PMOS TFT-type load cell has advantages of maintaining low stand-by current and stabilizing data maintenance, which are most important characteristics in the SRAM devices.
In such a PMOS TFT-type load cell, however, the channel of the PMOS is formed in a polysilicon film rather than in a single-crystal silicon film. Therefore, the PMOS TFT-type load cell inherently has drawbacks of low carrier mobility caused by a grain boundary potential barrier, thermal emission of carriers trapped by the grain boundary and leakage current produced by field emission, as well as a complicated process in manufacturing.
Accordingly, in order to solve the problems of low carrier mobility and leakage current caused by the grain boundary of the channel polysilicon film in a TFT technology, a method for minimizing the length of the grain boundary of the channel polysilicon film, i.e., a method for maximizing the grain size, has been suggested.
For this purpose, in a conventional technique, a channel of the TFT is formed such that an amorphous silicon film is deposited at a low temperature via a low pressure (LP) or plasma-enhanced chemical vapor deposition (PECVD) method, and an annealing is performed for polycrystalization under an inert nitrogen gas (N.sub.2) ambient at a relatively low temperature range of 600.degree.-650.degree. C. for long hours, thereby maximizing the grain size.
However, according to the above-stated method, the deposited amorphous silicon film is inevitably exposed to the atmosphere when it is transported to another tube, and the annealing for polycrystalization is carried out in another tube. Moreover, since the grain size of the finally-formed channel polysilicon film is smaller than 0.3 .mu.m, I.sub.on /I.sub.off current ratio among the channel characteristics of the TFT is low.